1. Field of the Invention
The present invention relates to a damper device and a turbomolecular pump having such a damper device, and more particularly to a damper mechanism for passively damping vibrations of a rotating member such as a rotating shaft of rotary machinery by consuming electromagnetic losses.
2. Description of the Prior Art
Damper devices which employ magnets have a long history, but much remains to be improved for preventing or damping vibrations with magnetic damper devices. When magnetic fluxes in magnetic materials are increased or reduced, they cause a hysteresis loss which is effective to damp vibrations. However, such a hysteresis loss has a small effect, and is effective to prevent or damp low-frequency vibrations only.
The effect of an eddy current loss only for preventing or damping vibrations will be described below. A damper device serves to convert vibration energy into thermal energy to damp applied vibrations. When a vibration is applied, it changes a magnetic flux distribution or density in an electric conductor or causes an electric conductor to cut a magnetic flux, resulting in an electromotive force generated in the electric conductor. The generated electromotive force occurs in a short-circuited loop in the electric conductor, which produces an eddy current I. If the short-circuited loop has a resistance R, then the electric conductor generates thermal energy represented by:
I.sup.2.multidot.R.multidot.
If the generated electromotive force is indicated by E, then the generated thermal energy may be expressed by: EQU E.sup.2 /R
Since the mechanical vibration energy is converted into thermal energy, the electric conductor operates as a vibration damper. In order to increase the efficiency of the vibration damper, it is necessary to increase the electromotive force E and reduce the resistance R.
One of the simplest forms of conventional vibration dampers is shown in FIG. 1 of the accompanying drawings. As shown in FIG. 1, the vibration damper comprises a magnet 1 and an electric conductor 2, one of which is fixed to a vibratable body and the other to a stationary body. The vibration damper damps vibrations applied thereto as follows: When the vibration damper is vibrated, the magnet 1 and the electric conductor 2 move relatively to each other in the directions indicated by the arrows, changing a magnetic flux path of magnetic flux .PHI. in the electric conductor 2 in accordance with the amplitude of the vibration. In other words, the electric conductor cuts the magnetic flux .PHI., or magnetic flux density at an area of the electric conductor 2 near the magnet 1 changes its magnitude in proportion to the amplitude of the vibration. When the magnetic flux density is thus changed, the electric conductor 2 develops an alternating electromotive force proportional to the change in the magnetic flux density to cause eddy currents. Thermal energy converted from the mechanical vibration energy is proportional to the square of the magnetic flux density. In order to increase the magnetic flux density in the electric conductor 2, it is necessary to either bring the electric conductor 2 closely toward the magnet 1 or reduce a gap if the electric conductor 2 is placed in the gap. Since, in either case, the conductor portion where the magnetic flux density is large, is small and a current I flows perpendicularly to the magnetic flux density in the small conductor portion, the path of the current I is inevitably narrow, equivalently resulting in an increase in the equivalent resistance R. If the gap of the magnetic flux path is widened to increase the volume of the electric conductor 2 which is placed in the gap, then the magnetic flux density B is reduced.
In order to increase the efficiency of the damper device, the conductor portion where the eddy current flows should be increased to reduce the resistance R. If the electric conductor is placed in the gap of magnetic flux path, then the conductor portion in the gap serves as a neck for increase of the eddy current. If the neck is enlarged, then the gap is widened, resulting in an increased magnetic resistance and a reduction in the magnetic flux density. For this reason, it is not preferable to provide a gap in the magnetic flux path and place the electric conductor in the gap.
It follows from the above considerations that it is preferable to provide a plurality of magnetic flux paths for a conductor circuit to choose from freely, and employ a structure which allows the magnetic flux paths to change greatly due to applied vibration. Therefore, a damper device is required to be of such a structure that longer and shorter gaps are produced when the magnetic flux paths and magnet are relatively displaced by the applied vibration, changing the magnetic flux paths greatly.
Turbomolecular pumps have moving impeller blades that rotate at high speed to exhaust a gas for thereby developing a vacuum. Some single-axis-controlled turbomolecular pumps include passive stable radial magnetic bearings having permanent magnets and active axial magnetic bearings. These magnetic radial and axial bearings support a rotatable shaft out of contact therewith to make the turbomolecular pumps operable at high speed and also make them free from contamination with lubricating oil.
Although the rotary assembly of such turbomolecular pumps is stabilized against radial movement and coning movement under magnetic forces of the permanent magnets, any attenuation of vibration of the rotatable shaft is very small. It is therefore necessary to employ a high-performance radial damper device in order to allow the rotatable shaft supported by the magnetic bearings to rotate at high speed. It is preferable that turbomolecular pumps have two passive radial magnetic bearings including permanent magnets at axially different positions, and these passive radial magnetic bearings be combined with respective damper devices for damping radial vibrations thereof.
FIGS. 2(A) and 2(B) of the accompanying drawings shows a known damper device 26 for use with such a magnetic bearing. As shown in FIGS. 2(A) and 2(B), the damper device 26 is of a laminated structure comprising an axially alternate assembly of damping plates 26a of rubber and metal plates 26b. Because of the laminated structure, the damper device 26 is not axially displaceable, but is radially movable for absorbing vibrations applied in the radial direction of the damper device 26. In turbomolecular pump applications where rotatable axis is vertical, both upper and lower magnetic bearings are combined with such damper devices for radially damping vibrations applied to the rotatable shaft at the upper and lower magnetic bearings. The upper and lower magnetic bearings should be capable of sufficiently damping vibrations in a full range of rotational speeds even when the vibration mode of the rotatable shaft, including a rotor, varies.
The damper device 26 shown in FIGS. 2(A) and 2(B) has some shortcomings. The damping plates 26a of rubber have their stiffness and damping effect progressively reduced as the temperature rises. Rubber has static and dynamic stiffnesses different from each other such that the static stiffness is a fraction of the dynamic stiffness. If the dynamic stiffness of the damping plates 26a is set to a value effective for attenuating radial vibrations, then the static stiffness of the damping plates 26a will be of a considerably small value. As a result, turbomolecular pumps which incorporate the damper device 26 with such a dynamic stiffness setting cannot be used in horizontally oriented installations, since static stiffness may be small.